Best material to minimize Impulse?

1. The problem statement, all variables and given/known data
Basically I want to know what material (That I could easily access) would be great to minimize impulse in the following situation:

A car is placed at the top of a ramp with some angle less than 45 degrees and at its front there is a tracker that measures the amount of force put into the car once it runs into the wall. I can add some material/compound to the front with the dimension 10x15x15 cm where 10 is the dimension in line with the length of the car.

(Foam is not allowed)

2. Relevant equations

p = mv
J = Ft

3. The attempt at a solution

Obviously I know that I need to minimize J (impulse) and maximize t (time), and I've thought of a few materials but I'm looking for any that you may have come across in your life that could help.